Several working hypotheses to explain the gamma-radiation-induced dissociation and reassociation of polysomes are being investigated in Tetrahymena and in mammalian cells in order to provide a detailed understanding of radiation-induced modifications in protein synthesis. Such an understanding will be helpful in explaining the diverse effects reported in the literature on the requirements for protein synthesis for repair of several kinds of radiation damage and on the various alterations of protein synthesis produced by irradiation of different types of cells. Some hypotheses being tested are: a radiation-induced decrease in the rate of initiation, a decrease in the rate of elongation or termination, release of large nuclear RNA, and/or a requirement for new RNA synthesis for reassociation. Experimental procedures employed include a direct assay of the rate of initiation of protein synthesis in intact cells, measurement of post-irradiation translation rate, determination of the weight average molecular weight of nascent peptides and newly-synthesized proteins, and analysis of the size distributions and types of post-irradiation RNAs contributing to the recovered polysomes.